Telecommunications - OSI Protocols - The physical
layer

The physical layer provides the mechanism for a DTE to transmit data to a DCE;
the data is seen as a stream of bits with no grouping into bytes.

MECHANICAL CHARACTERISTICS

These define matters such as the pin layout and the size and
shape of the latching block.

ELECTRICAL CHARACTERISTICS

These specify whether the connection is balanced or unbalanced,
the voltage levels used (and the tolerances), the input impedance's etc.

FUNCTIONAL CHARACTERISTICS

These characteristics are the ones which say what circuit performs
what function (possibly at what time). In general terms, there are four
broad categories of functions - data, control, timing and earth.

PROCEDURAL CHARACTRISTICS

These are the procedures required actually to use the interchange
circuits. There is still some debate into which layer of the model some
of these characteristics fit.

This layer defines the physical connection to the modem and may use V24, X21,
X21bis, V35, etc.

(V24 and X21bis are similar, use the same 25 pin connector, and have equal
status in ITU-T -CCITT).

The data is sent as a continuous serial synchronous or asynchronous data-stream.

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